1. Field of the Invention
This invention relates generally to navigation aids and devices, and particularly to a device for converting map bearings to provide an indication of the true land or sea bearing or position.
In reading a map and navigating with a compass, amateur or occasional navigators frequently fail to properly account for magnetic declination, either by neglecting it entirely, or by taking it in the wrong direction, thus producing an error of twice the magnetic declination. More seriously, amateur or occasional navigators may align the compass needle incorrectly or misinterpret the reading or make other similar errors to produce a 180 degree error.
There is a need for a navigation device which facilitates relatively foolproof conversion of map data into a land or sea bearing to be taken, and which permits determination of position by biangulation.
The principle of biangulation is of course well known, and involves taking bearings from two visible landmarks, and plotting those bearings back from the landmarks on a map, the intersection of the plots then indicating the person's position. Triangulation involves the same process, but is potentially slightly more accurate since averaging of three plots can be used. Unless very precise, such that all three plots intersect, triangulation will normally produce a very small triangle whose apexes are defined by the intersection points of the plots. For practical purposes, the person can assume that he or she is somewhere within that small triangle, if not at the center thereof.
2. Description of the Prior Art
Some prior art devices have used map transparencies mounted in a frame with a lens and rotatable grid, to facilitate obtaining a map bearing. For example, see U.S. Pat. No. 3,094,781 (Vangor, 1963).
However, such devices generally accomplish no more than can be accomplished using a protractor on a laid-out paper map, i.e. they merely provide a map bearing. There remains the problem of using a compass to convert the map bearing to the true land or sea bearing.
A navigation device similar to the present one is described in U.S. Pat. No. 5,060,390, by the same inventor. The device does not include the biangulation feature of the present invention, however.